The present invention relates to an exhaust purifying system and a method for controlling the exhaust purifying system. More particularly, the invention relates to an exhaust purifying system and a method for controlling the exhaust purifying system capable of avoiding occurrence of clogging, breakage, and the like of a reducing agent injection valve and a reducing agent supply path caused by the solidification of an aqueous urea solution.
Conventionally, the exhaust gas of an internal combustion engine installed in a vehicle includes nitrogen oxide (referred to below as “NOX”) or particulate matter (sometimes referred to below as “PM”). Of these substances, NOX is reduced by a urea selective catalytic reduction system to purify exhaust gas. The urea selective catalytic reduction system is configured to include a reducing agent supply apparatus, which supplies, from a reducing agent injection valve to an exhaust pipe, an aqueous urea solution functioning as a reducing agent pumped from a storage tank by a force-feed pump, and a selective catalytic reduction catalyst, which is one of exhaust gas purification catalysts capable of absorbing ammonia. Such a urea selective catalytic reduction system purifies exhaust gas by causing the selective catalytic reduction catalyst to absorb ammonia generated by decomposition of the aqueous urea solution and the NOX in exhaust gas to react with the ammonia in the selective catalytic reduction catalyst.
On the other hand, there is a diesel particulate filter (referred to below as a DPF) as an apparatus that collects PM and purifies exhaust gas. A DPF is disposed in the exhaust pipe of an internal combustion engine to collect PM in exhaust gas when the exhaust gas passes through the DPF. An exhaust purifying system including a DPF performs forced regeneration control as appropriate in which the PM accumulated on the DPF is forcibly burned by increasing the temperature of the DPF to approximately 500 degrees to 600 degrees to prevent clogging of the DPF.
Recently, as the purification standard of exhaust gas becomes severer, an exhaust purifying system including both a DPF and a selective catalytic reduction catalyst has come into widespread unit.
A urea selective catalytic reduction system is normally configured to recover the aqueous urea solution remaining in the reducing agent supply path when the internal combustion engine stops (see JP-A-2008-101564, for example). This avoids occurrence of clogging, breakage, or the like of the reducing agent supply path due to the freezing of the aqueous urea solution remaining in the reducing agent supply path.